Effect of size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys
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INTRODUCTION
DEPLETED uranium (DU) alloys and tungsten heavy alloys are two major materials currently used for penetrators. The former is processed in casting, whereas the latter is in powder metallurgy. The DU alloys are known to have better properties than tungsten heavy alloys, and their penetration performance is excellent over tungsten alloys by about 10 pct.[1–3] Due to environmental and health concerns over DU’s radioactivity, however, active research has been made recently to improve the properties of tungsten heavy alloys up to the level of DU alloys in order to replace DU alloys. The DU alloys have better penetration performance because of the “self-sharpening” effect; that is, adiabatic shear bands easily formed at the edge tip of a penetrator facilitate ready crack propagation along them, causing easy fall-off of edge parts. To improve the penetration performance of tungsten heavy alloy penetrators, recent studies focus on the process and microstructural factors to greatly promote the adiabatic shear banding as in DU alloys.[4–13] The formation of an adiabatic shear band is postulated as follows.[14] When plastic deformation occurs in a material, a large proportion of work done is converted into heat. If strain rate in a region is very high, there may not be enough time for the heat to diffuse away from the deforming zone. This effect would cause a local thermal softening effect. If the strength loss due to thermal softening becomes greater than the increase in strength due to strain rate hardening, plastic deformation will become unstable. Thus, homogeneous deformation will give way to a localized bandlike deformation mode. This is an adiabatic shear band. A prerequisite for adiabatic shear banding is a sufficient accumulation DONG-KUK KIM, Research Assistant, and SUNGHAK LEE, Professor, are with the Center for Advance Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. HEUNG-SUB SONG, Senior Research Scientist, is with the Agency for Defense Development, Daejeon, 300-600 Korea. Manuscript submitted April 7, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
of localized plastic work in a time that is shorter than that required for heat diffusion away from the plastic zone. However, the formation of adiabatic shear bands in tungsten heavy alloys is not as active as in other metals. Once they are formed though, they are wide and weak in general because they are composed of hard tungsten particles (bcc structure) and soft matrix (fcc structure).[15] Thus, microstructural factors such as the shape and size of tungsten particles; mechanical properties such as hardness, strength, elongation, and fracture toughness; dynamic deformation; and fracture behavior should be analyzed in detail to enhance the self-sharpening effect. Especially, the size and shape of tungsten particles are the microstructural factors of prime importance. It is reported that mechanical properties of tungsten heavy alloys are improved when tungsten particles are modified fine and spheroidal with redu
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